Members of the Myc family of cellular oncoproteins appear to play pivotal roles in processes of growth, differentiation,and programmed cell death. Regulation of these Myc-responsive activities is brought about by a complex strategy that is executed,in part,by interactions among a family of highly-related basic region helix-loop-helix leucine zipper (bHLH/LZ) proteins. This interacting network of bHLH/LZ proteins consists of Myc,Max,Mad,and Mxi1. In addition to being an obligate partner in the transcriptionally competent Myc/Max complex,Max also appears to function as a repressor in the form of a transcriptionally inert Max/Max homodimer or Mad/Max and Mxi1/Max heterodimers. We have isolated the murine homologs of mad and mxi1 and have determined their gene structure,developmental expression,and anti-oncogenic activities. Our studies provide clear biological evidence for a model of Myc regulation in which relative intracellular levels of Myc versus Mad or Mxi1 serve to control Myc- responsive activities. Specifically, we have demonstrated that (i)over- expression of mad and mxi1 can profoundly suppress myc-induced cellular transformation, (ii)the mxi1 gene maps to a chromosomal region that is known to be involved in several human cancers,(iii)a small amino-terminal domain of Mxi1 is responsible for its repressive functions,and (iv)opposing patterns of myc versus mad and mxi1 expression are apparent as mouse tissues and cultured cells progress towards growth arrest and terminal differentiation. Despite similarities between mad and mxi1 ,differences in their anti-oncogenic potencies,tissue-specific patterns of expression,and chromosomal localization are consistent with separable physiological functions. The goals of this proposal include (i)to determine the common and distinct activities of Mxi and Mad in mouse development,and (ii)to assess the impact of loss of mad or mxi1 function in malignant pathogenesis in vivo. To accomplish these, we will exploit the gene targeting approach to generate mice that are deficient for Mxi and Mad. The analysis of these mouse models will focus on whether loss-of- function leads to abnormalities in Myc-responsive activities (cell growth,differentiation,apoptosis) and/or to an enhanced predisposition to malignancy. An additional objective of this proposal is to elucidate the structural basis for the repressive activity of Mxi1. Mutations of the amino terminal repressive region will be assayed for alterations in anti- oncogenic potency in the REF assay. These cell culture-based studies will be complemented by a search for cellular proteins that interact with the Mxi1 repression domain under specific physiological circumstances.